Conventional automotive vehicle powertrains typically have multiple-ratio transmission mechanisms that establish power delivery paths from an engine to vehicle traction wheels. Adequate powertrain performance for heavy-duty or medium-duty powertrains requires more than one range of gear ratios so that the speed and torque characteristic of the engine will provide optimum traction wheel power throughout the operating speed range for the vehicle. Ratio range gearing, therefore, typically is used with a transmission mechanism in such high torque capacity powertrains so that sufficient overall gear ratio coverage is provided.
One class of transmission mechanisms in automotive vehicle powertrains includes countershaft gearing having power delivery gears journaled on a transmission mainshaft and countershaft gear elements journaled on a countershaft arranged in spaced, parallel disposition with respect to the mainshaft. The countershaft gear elements typically mesh with the gears mounted on a mainshaft axis. Controlled clutches, which may be either positive drive dog clutches or synchronizer clutches, selectively connect torque transmitting gears on the mainshaft axis to the mainshaft, thereby establishing a power delivery path from the engine to the traction wheels. The controlled clutches may be operator controlled, computer controlled, or automatically controlled. In other countershaft transmission mechanisms, the dog clutches or the synchronizer clutches may be mounted on the countershaft axis to selectively engage countershaft gear elements with torque transmitting gears on the mainshaft.
It is known in the art to use a dual clutch arrangement for selectively connecting the engine to first and second torque input gears, sometimes called headset gears, of a countershaft transmission mechanism. A dual clutch arrangement makes it possible for a countershaft transmission mechanism to be powershifted from one ratio to another. As one of the dual clutches is engaged, the other is disengaged. The engaged clutch will establish a power delivery path through the countershaft gear elements and through the main transmission gears as one or more of the ratio change clutches are engaged. A ratio change clutch for a main transmission gear, or a countershaft gear element that is not involved in a given selected power flow path, can be pre-engaged in preparation for a ratio change to an adjacent ratio. When a ratio change is initiated, the ratio change clutch for one main gear or for one countershaft gear element is disengaged, and a power flow path is established by the preengaged ratio change clutch for a second main gear or a second countershaft gear element. The dual clutches of the dual clutch arrangement are alternately engaged and disengaged (“swapped” or “traded”) thereby providing a smooth transition from one ratio to an adjacent ratio in a seamless fashion.
If the powertrain includes multiple speed range gearing in a torque flow path from the multiple ratio transmission mechanism to the vehicle traction wheels, it is necessary with known heavy-duty or medium-duty powertrains for the torque flow path to be interrupted during a transition from one range to the other. That torque interruption may deteriorate the shift quality and cause drivability issues due to the loss of acceleration during the shift resulting in an un-smooth acceleration, loss of speed during the shift, and/or longer times to reach a desired cruise speed. Further, the time needed to execute a ratio change in the range gearing increases the time needed to execute a shift between the highest overall ratio for one range to the adjacent lowest overall ratio for another range.
The ability to shift a dual clutch transmission between two gear ratios during a tip-in event, such as when the accelerator pedal is engaged or the throttle is at least partially opened, is a powershift event. Torque interrupts during a shift by the transmission may be avoided if the transmission is designed such that it powershifts. Currently, dual clutch transmissions exists that partially powershift, or powershift through only a portion of the shifts such that there are still some remaining torque interrupts. However, it is difficult to design a transmission with the appropriate gear ratios and step sizes where all of the shifts are powershifts such that the transmission is fully powershiftable. As a dual clutch transmission becomes more complex and include a greater number of speeds, packaging concerns also arise. It becomes even more difficult to provide powershifting between all of the gear ratios such that the transmission is fully and continuously powershiftable with no torque interrupts for a shift while minimizing a part count for the transmission to meet packaging and other requirements.